Seeking 318 rebuild advice

I tend to agree with the H or X pipe statement as well. But if you don't want duals for whatever reason, (I'm betting this will be an unpopular statement) and you're considering using stock manifolds you could run dual 2 1/4" head pipes into a single 3"...new trucks use that setup and it works well for them.

Volumetrically speaking, you're not losing much; Flow wise, a single 3" pipe is more than enough to satisfy 400+hp LS series engines...and keep in mind, pipes don't care what engine is in front of them. Realistically speaking, you're only paying for 1 or 2 sticks of pipe vs 3 or 4 at the muffler shop, provided they can bend 3" pipes.

Lots of options...